The present invention relates to pneumatic systems for accurately measuring and indicating fluid pressure, and to more particularly such systems for remotely measuring the weight, volume or depth of liquids in large processing or storage tanks. Cross-reference is made to the later-filed copending application, Ser. No. 359,581, filed Mar. 18, 1982, and assigned to the same assignee as the applicant's assignee herein.
The type of system over which this invention is an improvement is illustrated in U.S. Pat. No. 3,161,051 issued Dec. 15, 1964 (the disclosure of which is incorporated herein by reference). It generally comprises a flush diaphragm-type sensing unit or transmitter mounted in the tank wall (near the bottom of the tank), one side of the diaphragm being in contact with the liquid being measured. The other side ("indicator") side of the diaphragm is supplied a substantially constant volume of air (or other inert gas) which is vented through a nozzle to an extent proportionate to the position of the diaphragm. The result is that the pressure on the indicator side of the diaphragm is maintained substantially equal to the pressure on the liquid side of the diaphragm and the vent passage is constantly and dynamically proportional to the amount of such pressure (and hence the weight, volume or depth of the liquid in the tank).
One of the problems encountered with systems of this type is that they will not operate on tanks which are under a vacuum (such as in many "septic" processes in the food and pharmaceutical fields), and that they are inherently not very accurate when the pressure in the tank is very close to the vent or ambient pressure (such as when the tank is completely or almost empty). This problem is aggravated in systems using stainless steel diaphragms because for some reason it has been discovered that stainless steel diaphragms introduce significantly greater errors into the pressure readings at close to vent pressure than do conventional elastomeric diaphragms. This means that accuracy has to be sacrificed in applications requiring tough, cleanable stainless steel diaphragms.
It is therefore the primary object of the present invention to provide an improved system of the aforesaid type which has all the advantages of existing systems and yet overcomes the aforesaid problems. A related object resides in the provision of such a system which is very simple in construction, which does not materially increase operating costs, and which is readily adapted to be easily retrofit into existing systems with a minimum of difficulty.
These and other objects, features and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.